Though it is well established that there is a significant diurnal variation in the peripheral CD4+ T cell concentration (1), the reason for this variation has not been clearly defined. The general consensus is that it is meditated by redistribution of the T cells between the intra- and extra-vascular compartments (2). Our aim is to clarify the molecular basis for the diurnal variation in CD4 count by studying the expression of key adhesion molecules which are heterogeneously expressed on T lymphocytes, and are known to be important in T cell migration.